I think that's why GGP's comment ("Apple's approach is they have essentially hijacked the SMS protocol") is justified. If you want to use any iMessage functionality, you surrender your ability to send SMS to iOS users.
It surely doesn't warrant a per-message-toggle and a global setting seems reasonable in my opinion.
At the time I send a message to an iOS user (who has iMessage enabled), there's no way (AFAICT) for me to choose whether that message is sent via SMS or via iMessage.
(Perhaps if I turn off my data connections, iOS will eventually send the message via SMS?)
My wife and I use Signal to message each other and we have Signal as sms handler on our android phones. For some reason, she had to use a dumb phone with no Signal app for a couple of weeks. After a few days, she was suspicious that she is not getting some sms'es. I wanted to test if this was true, but I couldn't. Any message I sent to her number through Signal would be sent as a Signal message which wouldn't be delivered and I couldn't figure out how to send as sms. I had to change my sms app to another app, and then send using that app. Now, the new sms app couldn't read all older sms because Signal removes all sms from android sms database into Signal. So, now I use separate apps for Signal and sms.